The present invention relates to a plastic separation method for separating crushed waste including plural types of plastic pieces in a mixed state according to the type of plastic.
In recent years, recycling of waste has rapidly been promoted. Plastics consumed as materials for plastic products include vinyl chloride resins (hereinafter referred to as xe2x80x9cPVCxe2x80x9d), polyethylene resins (hereinafter referred to as xe2x80x9cPExe2x80x9d), polypropylene resins (hereinafter referred to as xe2x80x9cPPxe2x80x9d), polystyrene resins (hereinafter referred to as xe2x80x9cPSxe2x80x9d) and methacryl resins [acryl resins] (hereinafter referred to as xe2x80x9cPMMAxe2x80x9d), which account for about 80% of the total. Plastic wastes to be collected are mostly of these resins. Further, polyethylene terephthalate resins (hereinafter referred to as xe2x80x9cPETxe2x80x9d) which are mainly used as a material for PET bottles are collected in a unique manner. For recycling of these resins, it is important to separate the resins according to the type thereof.
One of prior art techniques for separating crushed plastic pieces is a plastic separator utilizing the electrostatic properties of the plastic pieces. An explanation will be given to the plastic separator S with reference to FIG. 6.
In the plastic separator S, crushed plastic waste including plastic pieces 1 of different types of resins to be separated are mixedly fed into a hopper 2. Plural types of plastic pieces 1 in a mixed state are fed into a triboelectric charger 3 from an outlet of the hopper 2, and agitated to be triboelectrically charged in the triboelectric charger 3. Then, the plastic pieces are spread onto an upper surface of a metal drum electrode 5. The metal drum electrode 5 is rotatable about a horizontal axis in a predetermined direction, and grounded.
A high voltage electrode 6 of an arcuate plate is disposed diagonally above the metal drum electrode 5 as extending in the direction of the rotation of the metal drum electrode. An electrode, e.g., a cathode, of a high voltage power source 7 is connected to the high voltage electrode 6, and an anode of the high voltage power source 7 is grounded. With this arrangement, the metal drum electrode 5 constitutes a rotary ground electrode, so that a separation electrostatic field is generated between the high voltage electrode 6 and the metal drum electrode 5.
A first separation vessel 8 and a second separation vessel 9 each having an open top are disposed below the metal drum electrode 5 in this order on the upstream side with respect to the rotation direction. A brush 10 is provided adjacent an outer circumferential surface of the metal drum electrode 5 for scraping plastic pieces 1 attached on the circumferential surface of the metal drum electrode 5.
An explanation will be given to a method for the separation of the plastic pieces 1 which employs the aforesaid construction. The mixed plastic pieces 1, i.e. the plural types of plastic pieces 1 are fed into the triboelectric charger 3 from the hopper 2. Then, the different types of plastic pieces 1 are agitated in a cylindrical body of the triboelectric charger 3 thereby to be brought into friction. Thus, the plastic pieces 1 are triboelectrically imparted with different amounts of charges having different polarities on a piece-by-piece basis. Then, the triboelectrically charged plastic pieces 1 are spread onto the metal drum electrode 5. Plastic pieces 1 negatively charged in the triboelectric charger 3 are repelled by the high voltage electrode 6 and attracted onto the metal drum electrode 5, and then allowed to fall into the second separation vessel 9 by the rotation of the metal drum electrode or scraped down from the surface of the metal drum electrode 5 into the second separation vessel 9 by the brush 10. On the other hand, plastic pieces 1 positively charged are attracted toward the high voltage electrode 6 and allowed to fall into the first separation vessel 8 by the rotation of the metal drum electrode 5.
As described above, the prior art triboelectric charger 3 is adapted to agitate the plastic pieces 1 of the plural different types of resins fed into the hopper 2 to triboelectrically charge the plastic pieces. If differences in amount (or total surface area) between the different types of plastic pieces 1 are too great, however, the plastic pieces 1 are not necessarily charged according to so-called triboelectric series during agitation, so that a required amount of charges (electrostatic potential) having a required polarity cannot be imparted to each of the plastic pieces for the separation. In such a case, it is impossible to properly carry out the separation by means of the metal drum electrode 5 and the high voltage electrode 6 by utilizing the charges imparted to the plastic pieces 1. It is therefore an object of the present invention to provide a plastic separation method which can solve the aforesaid problem.
Where the amount of a particular plastic to be separated is very small, the triboelectric charging is not necessarily carried out according to the triboelectric series on the basis of the type of plastic, so that the amount of charges imparted to the plastic is not sufficient for electrostatic separation. The invention solves this problem by increasing the amount of the particular plastic to such a level that the electrostatic separation can properly be performed. However, the particular plastic is present in a given amount, which cannot be changed as desired. Therefore, a plastic separation method is provided, in which a plastic of the same type as the particular plastic is added as a triboelectrification agent and, after the plastic is triboelectrically charged, the agent is not transferred to a separation process but left in a triboelectric charger for repeated use thereof.
More specifically, the present invention provides a method of separating pieces of a particular plastic from a mixture of plural types of crushed plastic pieces, the method comprising the steps of: feeding the crushed plastic pieces into a triboelectric charger; agitating the plastic pieces in the triboelectric charger for triboelectrically imparting the plastic pieces with different amounts of charges having different polarities depending on the type of plastic; and electrostatically separating the plastic pieces in an electrostatic separating section; wherein pieces of the particular plastic in a predetermined granular shape or plastic pieces of the same type as the particular plastic are additionally fed as a triboelectrification agent into the triboelectric charger for triboelectrically imparting the particular plastic pieces with a polarity and an amount of charge which allow for separation of the particular plastic pieces in the electrostatic separating section, if the weight of the particular plastic pieces is insufficient as compared with the other plastic pieces, i.e., smaller than 10% of the total weight of the plastic pieces.
The weight of the triboelectrification agent to be added is determined so as to satisfy the following expression:
(Wt+Wm)/W=0.1 to 0.9
wherein Wt is the weight of the particular plastic pieces in the triboelectric charger, and W is the total weight of the plastic pieces including the triboelectrification agent fed into the triboelectric charger.
The triboelectrification agent has a size which is one half to twice the size of the plastic pieces.
In accordance with the present invention, the addition of the triboelectrification agent makes it possible to assuredly separate the particular plastic pieces even present in a relatively small amount in the electrostatic separating section for recovery of the particular plastic pieces.
Further, the diameter of the triboelectrification agent is determined so as to be greater than the size D of the plastic pieces. Hence, the plastic pieces and the triboelectrification agent can be separated according to the size, so that the triboelectrification agent can be left in the triboelectric charger after the triboelectric charging for repeated use thereof in the triboelectric charger.
Alternatively, the diameter of the triboelectrification agent is determined so as to be different from the size of the plastic pieces. After the triboelectric charging, the plastic pieces and the triboelectrification agent are all expelled from the triboelectric charger, and separated according to the size. Then, the triboelectrification agent is fed back into the triboelectric charger so as to be repeatedly used.
Thus, the repeated use of the triboelectrification agent can be achieved for better economy.